In addition to the API-520 II advise provided by Latexman, There is similar mention also in current ASME VIII (2019) Non-Mandatory Appendix M-11 (see attached).
Since you advise that you have such an existing installation, it is is strongly recommended that you;-
* Check that such an installation has been reviewed and allowed by the manufacturer
* That documentation exists that the PRV installation has had proper assessment according to the noted API/ASME sections
The manufacturer should have been advised of the set up and also offered their advice and possible modification. For example;-
- Testing of the valve would have been done with the valve in this orientation
- Depending on application, some internals of the PRV may have needed modification (eg. guiding materials)
- Depending again on orientation, valve design and application, the PRV may require draining points
- Additional instructions in maintenance manual to cover such installation
You may need to address these issues through the pump supplier if the PRV came fitted already on it.
Per ISO-4126, only the term Safety Valve is used regardless of application or design.